In the case of a perpendicular magnetic recording method, a demagnetizing field in/around the recording layer of a subject medium functions differently from that of the conventional longitudinal recording method due to the magnetization recorded in the recording layer of the medium. Consequently, the demagnetizing field generated from the previous magnetization or the like also comes to affect a magnetic field (recording magnetic field) generated from the recording head differently from that of the longitudinal magnetic recording method. Hereunder, a description will be made for the measures to be taken for such a difference in the ways of influence between those two methods and problems that arise from the measures by picking up an NLTS (Non-Linear Transition Shift) in which the influence difference appears remarkably.
The NLTS is a phenomenon in which a demagnetizing field generated from the previous magnetization strengthens or weakens a recording magnetic field, thereby magnetization transition appears before or after the position at which the magnetization transition is to appear originally. As described in “IEEE Trans. Magn., 34, 1955 (1998)”, it is well known that in the case of the longitudinal magnetic recording method, the NLTS shifts magnetization transition that is about to occur forward from the position at which the magnetization transition is to occur originally due to the adjacent previous magnetization transition, thereby the bit length to be recorded on the subject medium is shortened than the original length (FIG. 1). On the other hand, each of the “IEEE Trans. Magn., 32, 3926 (1996) and the “IEEE Trans. Magn., 38, 1664 (2002)” reports that in the case of the perpendicular magnetic recording method, the NLTS shifts magnetization transition backward from the position at which the magnetization transition is to occur originally due to the adjacent previous magnetization transition, thereby the bit length becomes longer (FIG. 2).
The NLTS is a non-linear distortion of a signal in the time axis direction and it affects significantly the error rate at the time of data reading if it is neglected. And the NLTS is a phenomenon to occur due to a demagnetizing field generated from the previous recording magnetization, so that the influence of the NLTS depends on the previous recording magnetization pattern, that is, a recording data pattern. At present, the NLTS is compensated with use of a method referred to as write pre-compensation, which shifts an inverting position of the subject recording current beforehand, thereby shifting the position of the magnetization transition to improve the error rate significantly at the time of data reading. Concretely, in the case of the longitudinal magnetic recording method, as shown in FIG. 3, to compensate the NLTS that occurs due to the adjacent magnetization transition, the inverting position of the recording current is shifted backward according to the amount of the NLTS. Also in the case of the perpendicular magnetic recording method, the write pre-compensation is considered to be an important technique, but the inverting position of the recording current is required to be shifted in the opposite direction (forward) as shown in FIG. 4, since the NLTS direction is inverted from that of the longitudinal magnetic recording method. Thus the minimum inverting interval of the recording current becomes shorter than the expected shortest bit length in some cases. This means that the recording current frequency can become higher than the expected (calculated from the shortest bit length) maximum frequency, although it is locally/temporarily.
Japanese Patent Publication No. 2004-30730 gives means for absorbing an individual difference of a general head pre-amplifier with respect to the recording frequency of a recording current response for any of the longitudinal magnetic recording method and the perpendicularly magnetic recording method. Japanese Patent Publication No. 2005-18833 proposes a method for recording information on a medium only with use of a pulse-like overshoot current in the end.